The present invention relates to a parallel multi-inverter system including a plurality of pulse width modulation inverters which can suppress harmonic components of an output voltage and also can prevent a circulating current flowing through reactors connected between the inverters, and also relates to a motor drive system using the parallel multi-inverter system.
When it is desired to control the rotational speed of an AC motor, a pulse width modulation (PWM) inverter system is often employed. The PWM inverter system, however, has such a problem that since the output voltage of the PWM inverter system has two levels of positive and negative, harmonic components contained in the output voltage are much.
For the purpose of avoiding the above problem, there have been suggested systems for connecting a plurality of inverters on a multiple basis to eliminate such harmonic components from the output voltage. For example, one of such conventional harmonic eliminating systems is disclosed in JP-A-60-98875 wherein a plurality of voltage-source PWM inverters are connected in parallel to each other through a plurality of parallel reactors so that the phase of a carrier signal for controlling the respective inverters is sequentially shifted by 360.degree./n (n: the number of inverters) to thereby reduce side bands harmonic components of the carrier signal.
Also proposed in JP-A-64-47277 is such a system that a modulation signal having two output levels of positive and negative with respect to a reference potential is generated, first and second carrier signals respectively having amplitudes positive and negative with respect to the reference potential are generated, and the first and second carrier signals are compared with the modulation signal, so that a plurality of switching elements connected in series with each other are driven in accordance with a signal indicative of the comparison results to thereby remove harmonic components.
The above prior art systems can suppress the harmonic components of respective phase voltages, but there has not been suggested yet such a parallel multi-inverter system that a plurality of unit inverters each made up of a plurality of switching elements are connected in parallel to each other through parallel reactors so as to reduce the harmonic components of line-to-line voltages of the system and also to prevent a circulating current from flowing through the parallel reactors.